In process control systems, such as, for example, pipelines, that carry gaseous fluids, such as, for example compressed air, it is often useful to determine internal sound pressure levels. Internal sound pressure levels may be used to make transmission loss predictions, and can therefore help in the design and operation of pipelines and other process control systems.
In particular, aerodynamic noise that may be generated by control valves and propagate downstream in piping may lead to unacceptable external noise levels. Accordingly, prediction of such noise levels is important in order to ensure that such noise levels do not create undesirable or hazardous conditions, such as exceeding local, state or federal limits on radiated noise or causing hearing damage to people in the vicinity of a pipeline or structural damage to the pipeline.
Control valves may be installed in many applications involving the flow of fluids (gases or liquids) from one process to another, with the control valve commonly used to regulate the flow of fluid from one pipeline to another. A typical piping system will have a length of pipe upstream of a control valve, and a length of pipe downstream of the control valve.
The process conditions present upstream and downstream of a control valve will be dictated by various factors, such as the type of process medium that is flowing through the pipeline, the fluid dynamics of the control valve and the process control system in which it operates and for the operating conditions of the process control system. Some of the process conditions that are typically set upstream and downstream of the control valve are pressures and mass or volumetric flow rates, with the conditions downstream of the control valve being susceptible to creating undesirable noise radiating from the pipeline.
Being able to understand the transmission of sound pressure through pipe walls is an important part of predicting the sound pressure levels generated by control valves disturbing the flow and having that disturbed flow entering the downstream piping. The sound pressure level may be measured at some reference point that would be downstream of the control valve and some distance away from the pipe wall. Since the control valve will produce some level of noise, and the process plants in which they are installed typically have to comply with some overall sound pressure level regulation (e.g., OSHA regulations and/or or municipal ordinances), being able to predict the sound pressure levels radiating from piping downstream of control valves is also very important to users of process control equipment.
In particular, fluid flow downstream of a control valve may have a turbulent flow field that causes a pipe carrying the fluid to vibrate and radiate sound to the external environment.
In the past, noise predictions have been made using devices such as microphones that are placed near the flow passage of interest (e.g., in the vicinity of a pipe). Some past systems have used free field microphones external to the pipe and pressure transducers mounted internal to the pipe wall to estimate transmission losses. However, such arrangements can be expensive to install and maintain, thereby adding to the cost of an overall process control system.
Use of free field microphones and pressure transducers can be problematic in some situations, due to the cost and complexity of installing such devices. For example, installation of a pressure transducer into an internal pipe wall requires drilling a hole through the pipe wall and welding and centering the pressure transducer within the drilled hole, which may require a separate pressure-retaining fixture re-fabricated and installed on the pipeline.
There are several current methods in the literature to calculate a transmission loss value. One method that is used by control valve vendors is the International Electrotechnical Commission (IEC) method (Reference 10). The IEC control valve standard number 534-8-4 may be used to predict externally radiated noise one meter from a pipe wall, one meter downstream of the control valve outlet, and utilizes highly simplified procedures for calculating transmission loss. The IEC standard has procedures that are specifically tailored to control valve noise based on transmission loss at a single frequency, and are unsuitable for more general predictions of piping system noise. The IEC standard was largely developed for consistency in the reporting of control valve noise levels by competing vendors.
It should also be noted that IEC noise prediction only predicts one transmission loss value and internal sound pressure level at a calculated peak frequency. The transmission loss is a function of frequency and not just the peak frequency, so the IEC method will be limited when one is concerned with a frequency range.
This disclosure is directed to overcoming one or more of the problems or disadvantages associated with the prior art.